1. Field of the Invention
This invention relates generally to a monitoring and control assembly for an optical system, and more particularly to a monitoring and control assembly for a laser system.
2. Description of Related Art
Optical fiber communication systems provide for low loss and very high information carrying capacity. In practice, the bandwidth of optical fiber may be utilized by transmitting many distinct channels simultaneously using different carrier wavelengths. The associated technology is called wavelength division multiplexing (WDM). In a narrow band WDM system 8, 16 or more different wavelengths are closely spaced to increase fiber transmission capacity.
The wavelength bandwidth that any individual channel occupies depends on a number of factors, including the impressed information bandwidth, and margins to accommodate for carrier frequency drift, carrier frequency uncertainty, and to reduce possible inter-channel cross-talk due to non-ideal filters.
To maximize the number of channels, lasers with stable and precise wavelength control are required to provide narrowly spaced, multiple wavelengths. Some laser sources, for example distributed feedback (DFB) lasers, exhibit wavelength drift over time, in excess of the requirements for narrow band WDM. The wavelength of the device tends to change with aging under continuous power. Since telecommunication systems are expected to have a lifetime of the order of 25 years, wavelength control must be added to the laser transmitter to ensure minimum cross-talk between narrowly spaced channels over extended time periods.
Single wavelength optical communications systems are widely used in the industry. Ideally, systems designers seek minimum disruption of existing systems and compatibility with existing packaging in development of WDM systems.
Typically, known laser wavelength monitoring and stabilization systems are based on a unit external to the standard package of a laser source (transmitter). One commercially available system for monitoring and control of the wavelength of a semiconductor laser is an assembly based on crystal gratings. For example, in a known system manufactured by Accuwave, and described in the product literature, a wavelength locker unit is provided which comprises a lithium niobate crystal in which two Bragg gratings are written, illuminated by a collimated beam from a laser source coupled to the assembly, and two photodetectors. Each grating has a slightly different Bragg wavelength and angle relative to the input beam. The output reflected from the gratings is directed to the two detectors and the differential output is used to provide feedback control to the laser. Wavelength stability of better than 10 pm can be achieved with the control loop. However, the locker utilizes a separate unit from the transmitter, and thus requires external coupling to the laser or light source. Moreover, the unit is designed for a specific wavelength, as specified by the grating parameters. Different units are required for different wavelengths.
Another known type of wavelength monitoring/control assembly is based on a fiber grating. For example, GB Patent Application No. 96/00478 filed Mar. 4, 1996 to Epworth et al., relates to an external cavity type laser whose external reflector is provided by a Bragg reflector located in an optical fibre butted to an anti-reflection coated facet of the semiconductor laser. The grating is placed far enough from the laser that the longitudinal modes are so closely spaced that the laser operates multimode with so many modes as to make mode partition noise negligible. Another GB Patent Application No. 95/19614.3, filed Sep. 26, 1995 to Epworth et al., relates to using a chirped fiber grating for equalization and laser frequency stabilization.
Another system for stabilization of a semiconductor laser is described in U.S. Pat. No. 4,309,671 to Malyon which uses a pair of matched photodiodes and two beam splitters. The first beam splitter and first photodiode monitor power, and a second beam splitter, a frequency dependent filter and second photodiode are used to monitor wavelength changes. The outputs of the matched photodiodes are fed via amplifiers to a subtractor amplifier and the output is fed as negative feedback to the amplifier controlling operation of the laser.
Other known systems are based on a filter element such as a Fabry-Perot etalon. For example, U.S. Pat. No. 5,331,651 to Becker et al. describes the use of a Fabry-Perot etalon for fine tuning in conjunction with a grating for coarse tuning of the output of a laser.
In a system described in U.S. Pat. No. 5,438,579 to Eda et al., a Fabry-Perot etalon is used with a single photodetector to generate a signal used to lock onto one peak of a semiconductor laser, requiring collimated beams. Hill et al., in U.S. Pat. No. 4,839,614 describe a system for referencing frequencies of radiation from multiple sources relative to a reference source, using a filter element such as a Fabry-Perot etalon and a corresponding plurality of detectors.
Another system for laser wavelength stabilization is described in U.S. Pat. No. 4,914,662 to Nakatani et al. which involves spectroscopically processing the output of a variable wavelength laser and measuring a spatial distribution using image processing apparatus, and then comparing the distribution to that of a reference light source of fixed wavelength. The latter image processing system is complex, and not readily compatible with providing a low cost, compact unit.
Japanese Patent Application 92-157780 relates to a frequency stabilizer for semiconductor laser, without using external modulating means, and is based on an inclined Fabry-Perot etalon on which the laser source is incident, and two photo-detectors to detect respectively the transmitted and reflected signals. By subtracting outputs of the two detectors, a signal is provided for controlling the oscillation frequency. Resonator length is altered by changing the inclination of the etalon to allow for tunability. The implementation of this system for minimum space requires using the FP at a relatively large angle, with decreased stability in terms of center wavelength and bandwidth. On the other hand, a small FP angle requires added components and space, as shown in FIG. 1B of this patent application. Also, independent detectors are used, with potentially different response and aging characteristics.
Consequently, various existing systems for wavelength stabilization are known using a crystal grating, fiber grating or etalon based arrangement. The grating based systems lack wavelength tunability and many systems are based on relatively large control units external to a packaged laser source, with concurrent coupling, space and power dissipation problems. While etalon based systems provide tunability, none of the known configurations are sufficiently compact to incorporate in known standard packages without disruption.
There is a need for a monitoring and control assembly for an optical system that is more compact in design, more robust, lower cost and has a higher level of integration.